In recent years, further development of commercial value of products has been required with the spread of EPS. In such development, there is a requirement for keeping an assist function in case of a failure even though the assist function has a time limit for operation. For keeping the assist function of EPS, a method is known wherein a plurality of state quantity detecting means (which are for example a steering torque sensor, a steering angle sensor, a motor rotation angle sensor, etc.) are provided and abnormal one of the condition detecting means is found by comparing detection signals issued from the state quantity detecting means.